1. Field of the Invention
The present invention relates to a zoom lens, a variable magnification group, a camera unit having a photographic lens using such a zoom lens, a digital camera, a video camera, and a portable information terminal unit as an information equipment having a digital image photographing function.
2. Prior Art
The digital camera market is being extraordinarily expanded. Photographing of digital image is extended not only to a digital camera but also to a portable information terminal unit of portable telephone, and demands of users for digital camera are multilaterally branched. Among them, high image quality and miniaturization are always desired by the users and are given much weight among the various desired items. For this reason, high performance and miniaturization are also required to be compatible in a zoom lens used as a photographic lens.
In view of miniaturization, first of all, it is require to reduce the total length of lens (the distance from the most object side lens surface to an image surface). Furthermore, in a camera unit of so called xe2x80x9ccollapsible mount stylexe2x80x9d that is contemplated for compactness at the time of receiving a lens, it is important to reducing the thickness of each group, which moves at the time of zooming, in the direction of optical axis in order to reduce the dimension at the time of reception.
Although various types are considered for the zoom lens for digital camera, there is a type which is appropriate for miniaturization, which comprises a first group with a negative focal length, a second group with a positive focal length, a third group with a positive focal length in turn from the object side, and an iris provided in the object side of the second group to move integral with the second group, wherein the second group monotonously moves from the object side to the image side at the time of variation of magnification from the short focus end to the long focus end, and the first group moves to correct the variation in position of an image surface accompanying the variation of magnification.
For example, although Japanese Patent Publication No. Hei. 10(1998)-039214 discloses all of the basic construction before everything else among those of which proposed the zoom lenses of the above type, it is not always sufficient in the aspect of miniaturization. Although a zoom lens improved over the above type and advanced in miniaturization is disclosed in Japanese Patent Publication No. Hei. 11(1999)-287953, the miniaturization is not sufficient and also aberration correction cannot to be said as sufficient because only one aspherical surface is provided in the second group. Although Japanese Patent Publication No. 2000-089110 discloses a zoom lens which is contemplated for good correction of aberration using the two aspherical surfaces in the second group as its embodiment 3, it is not always advantageous in miniaturization because the thickness of second group is large.
Also, because the examples of prior art which are disclosed in these publications are not appropriate in the construction of each group, in particular the second group, sufficient aberration correction do not take place and do not have a performance which is able to cope with an image sensing device of 2,000,000 to 4,000,000 pixels.
Considering the above problems, it is an object of the present invention to contemplate the miniaturization of zoom lens used for photographing a digital image while maintaining high performance thereof.
In addition, it is an object of the present invention to provide a zoom lens and camera having a definition corresponding to an image sensing device of 2,000,000 to 4,000,000 pixels.
Furthermore, because it is important to minimize a lens diameter (maximum light effective diameter) in order to avoid the large-sizing of lens barrel formed from a plurality of stages in the case of zoom lens used in the camera of so called collapsible mount style, it is an object of the present invention to provide a zoom lens, a camera and a portable information terminal unit which are sufficiently compact while having high performance and of which the lens diameter (maximum light effective diameter) of the first group amongst others is small.
In the zoom lens consisting of three groups of negative, positive and positive as in the present invention, in general, when magnification is varied from short focus end to long focus end, the second group monotonously moves from the image side to the object side and the first group moves to correct the variation in position of an image surface accompanying the variation of magnification. The second group takes charge of most of variation of magnification function and the third group is provided mainly to keep exit pupil away from the image surface.
In order to realize a zoom lens of high definition, of which various aberrations are small, the variation of aberration due to variation of magnification should be restrained to be small, and in particular a variable magnification group which performs substantial variation of magnification, i.e., the second variable magnification group which is the main variable magnification group is needed to be excellently aberration-corrected in the total range of variation of magnification. For the excellent correction of aberration of the second group, basically it is considered to increase the constituent number of second group but the increase of constituent number will increase the thickness of second group in the direction of optical axis, whereby it will be impossible to attain sufficient miniaturization and furthermore the increase of cost will be caused.
Although second groups consisting of not more than four lenses are known: that consisting of three lenses: a positive lens, a negative lens, and a positive lens in tarn from the object side; that consisting four lenses: a positive lens, a positive lens, a negative lens, and a positive lens in turn from the object side; and that consisting of four lenses: a positive lens, a negative lens, a negative lens, and a positive lens in turn from the object side, the present invention realizes a second group having a aberration correcting capability over them.
That is, in the present invention, a zoom lens comprising the first group with a negative focal length, a second group with a positive focal length, and a third group with a positive focal length in turn from the object side, and including an iris provided in the object side of said second group to be moved integrally with the second group, in which at the time of variation of magnification from short focus end to long focus end, the second group monotonously moves from the image side to the object side and the first group moves to correct the variation in position of image surface accompanying the variation of magnification, wherein the second group consists of four lenses: a positive lens, a negative lens, a positive lens and a positive lens in turn from the object side.
Due to the relationship that an aperture iris is located in the object side of second group, in the second group, as a lens of image side is more remote from the aperture iris, its abaxial light passes a place which is more deviated from the optical axis, thereby deeply taking part in correction of abaxial aberration. Although the second group has a symmetrical arrangement having two positive magnifications each being provided on each side of a negative magnification, the positive magnification of image side which deeply takes part in correction of abaxial aberration is divided in two lenses, whereby degree of freedom is increased and it becomes possible to excellently correct the abaxial aberration.
In order to execute more sufficient correction of aberration, it is preferable to satisfy the following conditional relationship.
0.9 less than (LPN/Yxe2x80x2) less than 1.4 
Wherein, LPN is the distance from the apex of object side surface of the most object side positive lens of the second group to the apex of image side surface of negative lens, which is the second from the object side of the second group, and Yxe2x80x2 is the maximum image height.
In the second group, both of the object side surface of the most object side positive lens and the image side surface of negative lens which is the second from the object side have small curvatures and largely reciprocate aberrations each other, thereby most largely contributing to correction of aberration. In order to perform excellent correction of aberration, the height of light passing these two surfaces. When (LPN/Yxe2x80x2) is 1.4 or more, the marginal height of on-axial light in the image surface of the negative lens, which is the second from the object side, becomes too small, whereby the correction of spherical aberration becomes difficult. In addition, this is unfavorable for miniaturizing the second group. Meanwhile, when (LPN/Yxe2x80x2) is 0.9 or less, the height of abaxial main light in the negative lens, which is the second from the object side, becomes too small, whereby the correction of astigmatism and coma aberration becomes difficult.
Furthermore, in order to perform excellent correction of aberration, it is preferable to have two or more aspherical surfaces in the second group.
By using two aspherical surfaces in the places which individually have different light passing modes, it becomes possible to enhance the degree of freedom of aberration correction. Also, in order to perform the most efficient aberration correction, it is preferable to form the most object side surface and the most image side surface in the second group as aspherical surfaces.
Because the most object side face of second group is adjacent to the iris, on-axial and abaxial light beams pass through the surface substantially without being separated, and the spherical surface provided thereon mainly takes part in correction of spherical aberration or coma aberration. Meanwhile, because the most image side surface of the second group is remote from the iris, the on-axial and abaxial light beams somewhat separately pass therethrough, and the aspherical surface provided herein contributes to correction of astigmatism and the like. Like this, by using two aspherical surfaces as the most object side surface and the most image side surface, each aspherical surface induces sufficiently different effects, whereby the degree of freedom in aberration correction is greatly increased.
In the zoom lens of the present invention, in order to excellently correct each aberration, it is preferable that the first group comprises at least one negative lens the larger curvature surface being faced to the image side, and at least one positive lens with the larger curvature surface being faced to the object side, in turn from the object side and the image side surface of said negative lens is an aspherical surface.
By making the first group in this construction, it is possible to reduce the curvature of an image surface, and by forming the surface of large refractive angle for abaxial light as an aspherical surface, it becomes to suppress the distortion aberration especially in the short focus end.
In particular, the first group may consist of three lenses: a negative meniscus lens with a prominence surface faced to the object side, a negative lens with the larger curvature surface being faced to the image side, and a positive lens with the larger curvature surface being faced to the object side in turn from the object side, wherein said the image side surface of said negative lens is an aspherical surface.
According to this construction, the capability of aberration correction is enhanced, and thus it is advantageous for widening angle of view.
The zoom lens of the present invention may comprise three groups, four lenses: a positive lens, a negative lens conjugated to the positive lens, a positive lens and a positive lens in tarn from the object side.
In the second group the object side surface of the most object side positive lens and the image side lens of the negative lens second from the object side reciprocate aberrations with each other as explained in the above, whereby the cement error (eccentricity and the like) sometimes greatly influences on the imaging performance. By conjugating these two lenses, it becomes possible to suppress the cement error itself to be small.
The zoom lens according to the present invention may comprise three groups of four lenses in the second group: a positive lens, a negative lens, a positive lens conjugated to the negative lens, and a positive lens in turn from the object side.
In the second group, the image side surface of negative lens second from the object side and the object side surface of third positive lens from the object side largely reciprocate aberrations if they are designed so that there is a small difference of curvatures between them. In such a case, the influence that these affect on the imaging performance of these two lenses may be increased but it becomes possible to suppress the cement error itself to be small by conjugating these two lenses.
In addition, it is possible to obtain a small sized camera of high image quality by mounting said zoom lens in the camera as a photographic optical system.
Furthermore, the xe2x80x9cvariable magnification group in a zoom lensxe2x80x9d of the present invention is xe2x80x9cthat constructed as the second group to perform substantial variation of magnification in a zoom lens comprising the first group with a negative focal length, the second group with a positive focal length, the third group with a positive focal length in turn from the object side, and an iris provided in the object side of the second group to move integral with the second group, wherein at the time of variation of magnification from the short focus end to the long focus end, the second group monotonously moves from the image side and the object side, and said first group moves to correct the variation in position of an image surface accompanying the variation of magnification.xe2x80x9d
The variable magnification group has a characteristic as follows.
Namely, the variable magnification group constructed as the second group comprises three lenses: a positive lens with the larger curvature surface being faced to the object side, a negative lens with the larger curvature surface being faced to the image side, and a positive lens arranged in tan from the object side and the most object side surface and the most image side surface are aspherical surfaces.
The variable magnification group defined in claim 11 is preferable if the thickness in the optical axis direction, LG2 and the maximum image height, Yxe2x80x2, to satisfy the following relationship:
1.0 less than (LG2/Yxe2x80x2) less than 2.5.xe2x80x83xe2x80x83(1) 
The variable magnification group is preferable if the curvature radii of the most object side surface and the most image side surface, respectively defined as R31 and R32, to satisfy the following:
xe2x88x920.4 less than (R31+R32)/(R31xe2x88x92R32) less than 0.0.xe2x80x83xe2x80x83(2) 
In addition, because the image side surface is an aspherical surface in this case, said curvature radius, R32 is a paraxial curvature radius.
In the variable magnification group, the most object side positive lens and a negative lens following this may be conjugated with each other to form a conjugate lens, or three lenses may be formed to be independent from each other.
The variable magnification group, also, has a following characteristic.
That is, in the variable magnification group constructed as the second group to perform substantial variation of magnification, the most object side surface and the most image side surface are aspherical and the thickness in the direction of optical axis LG2 and the maximum image height Yxe2x80x2, to satisfy the following condition:
1.0 less than (LG2/Yxe2x80x2) less than 2.5.xe2x80x83xe2x80x83(1) 
The variable magnification group may take a four lens construction comprising four lenses: a positive lens with the larger curvature surface being faced to the object side, a negative lens with the larger curvature surface being faced to the image side, a positive lens and a positive lens arranged in turn from the object side.
In this case, the four lenses of the variable magnification group may be independent from each other but may be integrated by joining a positive lens on most object side and a negative lens following it as a conjugate lens.
The zoom lens of the present invention comprises a first group with a negative focal length, a second group with a negative focal length, a third focal length with a positive focal length arranged in turn from the object side, and including an iris provided in the object side of the second group integral thereto, wherein at the time of variation of magnification from the short focus end to the long focus end, the second group monotonously moves from the image side to the object side to perform substantial variation of magnification and the first group corrects the variation in position of an image surface accompanying the variation of magnification, characterized that it uses the variable magnification group according to any of claims 10 to 17.
The first group in the zoom lens may comprise at least one positive lens with the larger curvature surface being faced to the image side and at least one negative lens with the larger curvature surface being faced to the object side arranged in turn from the object side and the most object side surface, wherein the image side surface of the most image side negative lens among the at least one negative lens located in the object side, is formed as an aspherical surface.
The first group in the zoom lens may comprise a negative meniscus lens with the convex surface being faced to the object side, a negative lens with the larger curvature surface being faced to the image side, and a positive lens with the larger curvature surface being faced to the object side arranged in turn from the object side, wherein the image side surface of the negative lens located second from the object side is formed as an aspherical surface.
In this case, it is preferable if the focal length of meniscus lens located in the most object side in the first group fL1 and the focal length of the negative located second from the object side of the first group fL2, to satisfy the condition as follows:
0.7 less than (fL1/fL2) less than 2.0.xe2x80x83xe2x80x83(3) 
In addition, the first group in the zoom lens may take a two lens construction comprising a negative meniscus lens with the convex surface being faced to the object side and a positive lens with the larger curvature surface being faced to the object side arranged in turn from the object side, wherein the image side surface of said negative meniscus lens is formed in an aspherical surface.
In the zoom lens in the other embodiment, the third group may consist of a positive lens with the larger curvature surface being faced to the object side, wherein the lens is adapted to have at least one aspherical surface.
The zoom lens preferably comprises eight or less constituent lenses in all of the system.
The camera unit of the present invention is provided with a zoom lens according to any of claims 18 to 24 as a photographic zoom lens.
The camera unit may take a construction that the photographic zoom lens is received in a collapsible mount style.
The camera unit may have a function for taking a photographic image as digital information and in this case, a light receiving device for receiving an image formed by the zoom lens may have 2,000,000 or more pixels.
The camera unit may be a portable information terminal unit.
In the present invention, the second group which is the main variable magnification group comprises a positive lens with the larger curvature surface being faced to the object side, a positive lens with a lager curvature surface faced to the image side, and a positive lens, wherein the most object side surface and the most image side surface are formed as asymmetric surfaces.
That is, the second group is made as a triplet type in which the correction of chromatic aberration and correction of the minimum number of compatible constituents are compatibly performed by the minimum number of constituents, whereby miniaturization is realized, and then the aberration correction with high degree of freedom is performed using two asymmetric surfaces, whereby high performance is attained.
Because the most object side surface is positioned xe2x80x9cadjacent to the iris,xe2x80x9d on-axial and abaxial light beams pass through substantially without being separated and the aspherical surface provided in this surface mainly contributes to the correction of spherical aberration or coma aberration. Because the most image side surface of the second group is separated from the iris, the on-axial and off-axial lights are somewhat separated and pass through it. Therefore, the aspherical surface provided in this surface contributes to the correction of astigmatism.
By using two aspherical surfaces as the most object side surface and the most image side surface to make the effects induced by each of aspherical surfaces, it is possible to greatly increase the degree of freedom in correction of monochromatic aberration, whereby it is possible to perform sufficient correction of various aberrations including chromatic aberration even with small number of triplet constructions.
Although the present invention is preferable and appropriate for miniaturization because it takes a three lens construction, it is preferable to satisfy the condition (1) in order to attain additional miniaturization. When the parameter, LG2/Yxe2x80x2 is the upper limit value, 2.5 or more, the thickness of the second group in the optical axis is increased, whereby sufficient miniaturization cannot be attained. To the contrary, when the parameter, LG2/Yxe2x80x2 is the lower limit value, 1.0 or less, xe2x80x9cthe most image side surfacexe2x80x9d of the second group approaches to the iris, the effect of aspherical surface for performing xe2x80x9cthe correction of astigmatism and the likexe2x80x9d cannot be sufficiently exhibited, whereby it becomes difficult to correct astigmatism and the like.
When the second group takes the three lens construction as explained in the above, additional miniaturization can be realized if it satisfies the following condition, which is narrower than the condition (2):
1.0 less than (LG2/Yxe2x80x2) less than 2.0.xe2x80x83xe2x80x83(1xe2x80x2) 
If the second group is constructed as triplet type, the object side surface (aspherical surface) of the most object side positive lens in the second group and the image side surface of the negative lens following it xe2x80x9cwill largely reciprocate aberrations with each other.xe2x80x9d Due to this, the influence that the cement error (eccentricity and the like) of these two lenses gives to the imaging function is frequently enlarged.
In this respect, it is possible to suppress the cement error itself to be small by conjugating these two lenses.
However, in the present invention, an aspherical surface is also employed in the positive lens of the most image side in the second group, the influence given on the imaging performance by the cement error of this positive lens is great. In order to suppress the influence on the imaging performance by the cement error of xe2x80x9cthe positive lens of the most image sidexe2x80x9d in the second group to be small, it is preferable to satisfy the condition (2).
The condition (2) is to complete the reciprocation of aberrations in the object side surface and image side surface (aspherical surface) of the positive lens of the most image side in the second group if possible, so that the positional relationship with another lens will not become so strict.
Because when the parameter, (R31+R32)/(R31xe2x88x92R32) is the upper limit value, 0.0, or more, the aberration generated in the image side surface of the positive lens of the most image side becomes larger than the aberration generated in the object side surface, and when it is the lower limit, xe2x88x920.4 or less, the aberration generated in the object side surface becomes larger than the aberration generated in the image side surface, whereby the reciprocation of aberrations between different lens surfaces will be increased in both cases, the influence on the imaging performance by the cement error of the positive lens (of the most image side in the second group) will be increased.
As explained in the above, if the present invention satisfies said condition (1) although it is of three-lens construction, the desires for high performance and miniaturization can be satisfied even if the second group takes other construction rather than the three-lens construction because the condition (1) limits the size of the second group.
That is, in this case, it is possible to employ four-lens construction comprising four lenses: a positive lens with the larger curvature surface being faced to the object side, a negative lens with the larger curvature surface being faced to the image side, a positive lens, and a positive lens arranged in turn from the object side like the variable magnification group, and also possible to integrate the positive lens of the most object side and the negative lens following this as a conjugate lens.
Although the zoom lens uses the variable magnification group as a variable magnification group of three-group construction as explained in the above, for more excellent correction of each aberration in the zoom lens, it is preferable that the first group comprises at least one positive lens with the larger curvature surface being faced to the image side and at least one negative lens with the larger curvature surface being faced to the object side arranged in turn from the object side and the most object side surface, wherein the image side surface of the negative lens located in the most image side in the at least one negative lens is formed as an aspherical surface.
By forming the first group in this construction, it is possible to reduce the curvature of an image surface, and by forming the surface having a large refractive angle of abaxial light in an aspherical surface, it becomes possible to suppress distortion aberration, especially in the short focus end.
In this case, if the first group in the zoom lens takes three-group construction comprising a negative meniscus lens with the convex surface being faced to the object side, a negative lens with the larger curvature surface being faced to the image side, and a positive lens with the larger curvature surface being faced to the object side arranged in turn from the object side, wherein the image side surface of the negative lens located second from the object side is formed as an aspherical surface like the zoom, and it satisfies the condition (3), it is possible to reduce the influence on the imaging function by formation error of said aspherical surface, whereby the effect of aspherical surface can be efficiently exhibited.
That is, in the condition (3), the parameter (fL1/fL2) indicates the magnification ratio of two negative lenses in the first group, wherein if this parameter is the upper limit value, 2.0 or more, the magnification of second negative lens provided with an aspherical surface is strengthened, whereby the degree of difficulty in the case of manufacturing the lens by forming (mold) is raised because the difference of thickness between the center and periphery of lens is increased, beyond that the influence on the imaging function by the producing error of aspherical surface. Meanwhile, if the parameter (fL1/fL2) is the lower limit value, 0.7 or less, the magnification of second negative lens provided with an aspherical surface is weakened and the refractive angle of abaxial light in the image side surface is reduced, the effect of aspherical surface is diluted.
In addition, more preferably, it is advantageous to satisfy the following condition which is narrower than the condition (3);
0.7 less than (fL1/fL2) less than 1.5.xe2x80x83xe2x80x83(3xe2x80x2) 
Furthermore, by constructing the first group in two lens construction comprising a negative meniscus lens with the convex surface being faced to the object side and a positive lens with the larger curvature surface being faced to the object side arranged in turn from the object side, like the zoom lens in one form, the image side surface of said negative meniscus lens may be formed as an aspherical surface, and by making the first group in such a two-lens construction, it is advantageous for miniaturization with more simplified construction. In addition, if the first group is made in the three-lens construction, it is advantageous to widen the angle of view because the capability for correction of aberration is enhanced.
It is preferable that the third group is formed from a positive lens with the larger curvature surface being faced to the object side, wherein at least one surface is formed in an aspherical surface. According to this construction, it is possible to more excellently correct abaxial aberration such as astigmatism while suppressing the thickness of the third group to the minimum.
Additionally, although the third group may be fixed at the time of variation of magnification, it is possible to increase the degree of freedom for correction of aberration by moving it a little.
Furthermore, in the zoom lens of this type with three-group construction, the abaxial light at the short focus end is most remotely deviated from the optical axis in the most object side surface. Therefore, the maximum outer diameter of entire zoom lens is determined by the light effective diameter at the short focus end of the most object side surface in the first group. That is, it is evident that reducing the lens diameter of zoom lens of this type will reduce the height of abaxial light in the most object side surface of the first group.
In order to make the height of abaxial light in the most object side surface in the first group, it is preferred to locate a negative refractive force in the object side and a positive force in the image side in the first group and to strengthen each refractive magnification. However, uselessly strengthening the refractive force makes it difficult to correct an aberration, thereby causing the deterioration of imaging performance.
Therefore, in the other embodiment, the present invention is constructed in such a manner that the first group comprises three lenses: a negative meniscus lens with the convex surface being faced to the object side, a negative lens with the larger curvature surface being faced to the image side, and a positive lens with the larger curvature surface being faced to the object side arranged in turn from the object side, wherein at least one of image side surfaces of negative meniscus lenses of first group is an aspherical surface and satisfies the following condition.
xe2x88x920.05 less than (Yxe2x80x2/R6) less than 0.05 
Wherein, R6 indicates the radius of curvature of image side surface of positive lens in the first group and  less than indicates the maxim image height.
When (Yxe2x80x2/R6) is 0.05 or more, a relatively strong negative refractive force is present in the most image side of the first group, whereby it becomes difficult to reduce the height of light of abaxial light beams in the most object side of the first group. In all of the prior arts, (Yxe2x80x2/R6) was 0.05 or more and thus sufficient miniaturization of the first group has not been made. Meanwhile, if (Yxe2x80x2/R6) is xe2x88x920.05 or less, curvature of an image surface is readily produced and thus it is difficult to secure the abaxial performance.
By forming a surface having a large refractive angle of abaxial light as an aspherical surface, it becomes possible to suppress the distortional aberration especially at the short focus end, and the light beams become thicker, whereby it is effective in correction of spherical aberration and coma aberration. If an aspherical surface is provided in the second negative meniscus lens, the outer diameter of aspherical lens is reduced and it is advantages in the aspect of machining and cost, as compared to the case where the aspherical surface is provided in the meniscus lens of the most object side.
That is, if at least one of image side surfaces of negative meniscus lenses in the first group is formed as an aspherical surface and at the same time the most image side surface of the first group is made to be of weak refractive force regardless of positive or negative, it is compatible to sufficiently reduce the diameter of the first group and excellently correct an aberration.
In addition, in view of the practical machining and measuring of lens, it is preferable if the image side surface of positive lens in the first group is flat, i.e., (Yxe2x80x2/R6)=0.
Using a so-called flat-convex lens having a flat side, there will be a merit in that a primary standard of flat side is not required or it is not required to provide a curvature on a polishing dish for machining a recess. Furthermore, although it is difficult to measure a surface accuracy in a surface having an excessively large radius of curvature because it is poorly matched with a reference light of conventional interferometer, if the surface is flatly formed, it becomes also easy to measure the surface accuracy because the reference light is to the a plane wave.
In order to further decrease the diameter of the first group, it is preferable to satisfy the following relation ship.
xe2x88x921.5 less than (fL3/fG1) less than xe2x88x921.0 
Wherein fL3 indicates the focal length of positive lens of the first group and fG1 indicates the focal length of the first group.
If (fL3/fG1) is set to be greater than xe2x88x921.5, a sufficiently strong positive refractive force is positioned in the image side of the first group, whereby it will be advantageous to decrease the light height of abaxial light beams in the most object side surface of the first group. However, if (fL3/fG1) is xe2x88x921.0 or more, the refractive force will be too strong, and thus the correction of aberration will be difficult.
In addition, if the image side surface of second negative meniscus lens in the first group is formed as an aspherical surface, it is preferable to satisfy the following conditional relationship.
0.7 less than (fL1/fL2) less than 2.0 
Herein, fL1 indicates the focal length of the negative meniscus lens of the most object side in the first group and fL2 indicates the focal length of second negative lens in the first group. This conditional relationship indicates a magnification ratio of two negative lenses, wherein if (fL1/fL2) is 2.0 or more, the magnification of second negative lens provided with an aspherical surface becomes strong, whereby the degree of difficulty in the case of manufacturing the lens by forming (mold) is raised because the difference of thickness between the center and periphery of lens is increased, beyond that the influence on the imaging function by the manufacturing error of aspherical surface. Meanwhile, if the parameter (fL1/fL2) is 0.7 or less, the magnification of second negative lens provided with an aspherical surface is weakened and the refractive angle of abaxial light in the image side surface is reduced, the effect of aspherical surface is diluted In addition, more preferably, it is advantageous to satisfy the following condition:
0.7 less than (fL1/fL2) less than 15. 
If refractive force of each lens is strengthen to a certain degree by firstly making the first group have a small size, deterioration of imaging performance by the offset of optical axes among lenses between caused at the time of assembling them in the lens frame tends to be serious. Like this, if the sensitivity of eccentric error is high, it is preferable that the relative eccentricity of negative and positive lenses in the first group is adjustable.
The present invention strengthens the refractive force of positive lens and further provides a negative lens with an aspherical surface, whereby the reciprocation of aberrations between the negative lens and positive lens is increased, wherein it is most efficient to adjust this position because the influence of relative eccentricity of negative lens and positive lens is especially increased. In particular, because an air lens which may be present between the object side surface of positive lens and the image side surface of second meniscus lens plays an important part in correction of aberration if the image side surface of second negative meniscus lens is an aspherical surface, it is preferable to perform the adjustment so that the spherical center of the object side surface of positive lens is positioned on the axis of aspherical surface.
In addition, in order to simplify the adjustment of relative eccentricity of negative lens and positive lens in the first group, it is preferable to make the positive stationary and two negative lenses movable in the direction perpendicular to the optical axis. Furthermore, it is preferable to make two negative lenses integrally adjustable. For example, it can be considered to construct in such a manner that the third lens L3, which is positive, is engaged with a stationary frame 21, and a movable fame 22 is movable to the stationary frame 21, wherein the first lens L1 and second lens L2, both of which are negative, are integrally engaged with the movable frame, as shown in FIG. 79. With this construction, because the movable part is positioned in the most object side of zoom lens, it is easy to externally adjust it or to fix it by adhesion and the like after the adjustment. In FIG. 79, the reference numeral 23 is the adhesion part between the stationary frame 21 and the third lens L3, the reference numeral 22a is a caulking, and the reference numeral 24 is the adhesion part after adjustment.
If said zoom lens is built in a camera or portable information terminal unit as a photographic optical system, it is possible to obtain a compact camera or portable information terminal unit of high image quality.